Linear oxymethylene polymers or copolymers having repeating oxymethylene units (--CH.sub.2 O--) are well known. The process for preparing such polymers or copolymers is roughly divided into two processes, the first being a polymerization process using anhydrous formaldehyde as the main monomer, while the second being a polymerization process using a cyclic acetal (e.g. trioxane, a cyclic trimer of formaldehyde) as the main monomer.
The polymerization or copolymerization of substantially anhydrous formaidehyde in the presence of an anionic or cationic catalyst has been proposed as the first process, and a process for polymerizing or copolymerizing a low cyclic acetal as the main monomer, such as trioxane, in the presence of a cationic catalyst has been practiced as the second process.
Generally, a polymer or copolymer prepared by such processes has a linear structure and a broad molecular weight distribution, and has been widely used as materials for engineering plastics.
Apart from such linear oxymethylene polymers, the inventors have found that a specific macrocyclic polyoxymethylene which has a relatively small molecular weight distribution and a molecular weight not more than 5000 can be obtained as a part of products in a cationic polymerization of a cyclic monomer such as trioxane. A technique to isolate the cyclic polymer was established also by the same inventors and proposed in Japanese Provisional Publication No. 55712/1990. However, in such a case, it is not economical to isolate the desired cyclic polymer in a large amount without investment of additional polymerization equipment because the desired macrocyclic polymer is produced in a small amount as a by-product during the polymerization process of trioxane or the like.
It has now been found that when a conventional linear oxymethylene polymer is treated with a cationic catalyst, there is obtained a cyclic polyoxymethylene polymer which is stable to alkali and has a relatively large ring structure (number average molecular weight being 500 to 5,000) and has no terminal groups.
The present invention is related to a process for preparing a cyclic polyoxymethylene which comprises subjecting an alkali-degradative linear polyoxymethylene homopolymer to a solid state cyclization reaction by treating it with a medium containing a cationic catalyst to thereby provide a cyclic polyoxymethylene having a number average molecular weight of 500 to 5,000, and treating the resultant reaction mixture with an alkaline medium to remove the unreacted linear polyoxymethylene homopolymer by hydrolysis to thereby isolate the desired cyclic polyoxymethylene.
The cyclic polyoxymethylene obtained by a process of the present invention is a specific polyoxymethylene having a relatively large ring structure (number average molecular weight of 500 to 5,000) and no terminal group, and is represented as the following general formula: ##STR1## wherein n=17 to 170
According to test obtained by means of NMR, infrared spectrophotometry, and GPC as well as a hydrolytic cleavage test by alkali, the cyclic polymer formed by the process of this invention has a relatively large ring structure, no terminal group and a relatively narrow molecular weight distribution.